Control circuitry for vehicle guidance mechanism

ABSTRACT

A vehicle guidance mechanism for guiding a vehicle along a guide line comprises a radiation source that causes the guide line to reflect or emit a guidance signal; photocell sensors mounted in the vehicle and adapted to sense the guidance signal and produce an error output signal proportional to the variation of vehicle position from a predetermined lateral position with respect to the guide line; and a control device in the vehicle adapted to steer the vehicle in response to the error signal so as to cause the vehicle to follow the guide line. An automatic contrast or gain control circuit eliminates the effect of background illumination in the output signal received from the guide line. Modulation circuitry and appropriate filters reduce the effects of static background illumination and improve the reliability and line detecting ability of the guide system. Line detection circuitry is employed to prevent operation of the automatic control device unless the vehicle is tracking a valid guide line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to line tracking systems for automaticallydirecting a vehicle along a predetermined path and more particularly toa control circuitry for a line tracking system capable of tracking aninvisible guide line.

2. Description of the Prior Art

A variety of automatic control devices for vehicles have previously beendeveloped. The principal use for such automatic guidance systems hasbeen in connection with industrial vehicles of the type that performdelivery or work functions in an industrial or office facility. Systemsalso have been developed for automobile guidance on highways but no suchsystems have been implemented on a commercial basis.

The basic types of guidance systems previously used on unmannedindustrial vehicles have been permanent tracks, buried wire, reflectedlight, and radio controlled.

Track vehicles are vehicles that ride along a fixed rail or a track andare guided by the contour of the track. These vehicles are usedprimarily for automated storage-retrieval systems such as lift trucksthat move on tracks between many-leveled storage racks and stock orretrieve material under computer control. This technique is unacceptablefor floor maintenance machines (i.e., machines that scrub, sweep, orvacuum a floor) or any other application except for highly structuredenvironments, such as stadiums, theaters, or stairwells. The presence ofa fixed or permanent track on many floor surfaces is aesthetically aswell as physically undesirable, and the installation of such a track isa substantial expense. Further, when a permanent track is installed, itis difficult and expensive to change the location of the track, therebyreducing the flexibility of the system.

Buried wire guidance systems have been applied successfully to variousmaterial handling systems, including hospital food trays, office mailcarts, and automotive parts for use on assembly lines. In such a system,a wire is buried in the floor along the track the vehicle is to follow,and both ends of the wire are terminated at an excitation source. Thewire is excited with an alternating current (typically 10 kHz) and itsradiation pattern is then detected by two sensing coils on the vehicle.The differential output of these coils is utilized to drive a steeringmechanism so that the vehicle is steered to follow the buried wire. Thistechnique has serious limitations, one of the principal limitationsbeeing that it is generally not practical to employ a buried wire systemin an existing structure. To apply a buried wire system to an existingfloor structure, it would be necessary to cut a long groove in the floorstructure along the path the vehicle is to follow and lay a new floorsurface over the wire after it is buried in the groove. Even if theexpense of burying a wire in an existing floor structure could bejustified for a given application, the system would lack flexibilitybecause to change the position of any given line, it would be necessaryto cut a new groove in the floor and lay a new wire in this groove,while at the same time disconnecting the old wire.

Other disadvantages with a buried wire system are that the materials arerelatively expensive, as well as being difficult to install and remove,and interference problems could be encountered when a buried wire systemis used in a floor that is heavily reinforced with metal reinforcingrods.

Buried wire guidance systems are not appropriate for floor care machinesfor all of these reasons--they are too expensive, and the systemcontains little flexibility with regard to changing the placement of theguide line.

In reflected light sensing systems, a brightly visible line thatcontrasts markedly with the surface of the floor is laid along apredetermined path to be followed by the vehicle. The vehicle includes alight source of visible light that shines on the floor and photocellsensors that detect the level of light reflected from the floor (i.e.,both the background and guide line illumination). The sensors areconnected to a differential output amplifier much in the manner of thesensing coils in the buried wire system. The differential output isconnected to a servo drive mechanism, which steers the vehicle along theguide line as the vehicle moves along the floor. One of the problemswith the reflected light system is that the background or spuriousillumination from the floor also is received by the photocells, and thisillumination impairs the ability of the system to detect the contrastbetween the guide line and the background illumination. With surfaceshaving different contrast levels, such as a tile surface employing blackand white checkered tiles, accurate tracking with a reflected lightsensing system is unreliable.

Indeed, reflected light guidance systems have been known to cause avehicle to follow a beam of sunlight that is cast across a floor surfacefrom a window. Such unreliability is a critical defect in automaticmachines because unreliable machines require constant supervision (thuseliminating labor cost savings), and an unmanned machine out of controlon a spurious or non-existent line can be extremely hazardous.

One of the positive attributes of a reflected light system is that anexisting line can be removed or moved to a new location easily, thusdefining a new guide line for the vehicle to follow. However, the veryimpermanence and visibility of such a guide line also has drawback. Avisible and removable guide line invites vandalism, and such guide linesare aesthetically unappealing. The aesthetic unacceptability along isfatal to the use of a reflected light system in many applications, oneof which is in the floor care industry. It would be completelyunacceptable to almost any operation to have a complete matrix of highlyvisible lines crisscrossing on the floor surface just so that a floorscrubber or a floor sweeper could be operated automatically.

Another problem with a reflected light tracking system is that suchsystems are effectively limited to use on hard, flat surfaces, such asconcrete or tile. Problems with obtaining the necessary reflectancelevels and with the aesthetic undesirability of a solid, bright visibleline on a carpet would prevent the application of such a system in anysuch environment.

Notwithstanding the fact that the visible line system and the othersystems designed above have been in existence for many years (visibleline systems having been in existence since at least 1937) heretofore nosystem has been developed which obviates the disadvantages of the threesystems discussed above and provides a suitable automatic guidancesystem for use in connection with floor care machinery or other types ofmachinery, wherein a high degree of system reliability is essential, andsystem flexibility for rerouting the guide path and line invisibilityare at least desirable. It is a principal object of the presentinvention to provide an automatic guidance system for a vehicle that ishighly reliable, and is capable of following a guide line that issubstantially invisible under normal ambient lighting conditions, andcan be applied and reapplied easily to the surface of an existing floor.

SUMMARY OF THE INVENTION

The present invention comprises an improved control circuit for avehicle guidance mechanism for automatically guiding a vehicle along aguide line. The vehicle guidance system comprises a radiation sourcethat causes the guide line to emit or reflect a guidance signal; sensorcircuitry that senses the guidance signal and generates an error signalproportion to the vairation of the vehicle position from a predeterminedlateral position with respect to the guide line; and a control mechanismthat steers the vehicle along the guide line in response to the errorsignal generated by the sensor mechanism.

In order to improve the effectiveness and reliability of the guidancesystem of the present invention, several important features areincorporated in the sensor control circuit for this system. One of thefeatures is a modulation circuit that eliminates the effect of staticbackground radiation from the sensor output signal. This circuit firstmodulates the radiation source with a predetermined modulation signal todistinguish the guidance signal produced by the radiation source frombackground radiation sources. The modulation circuit then demodulatesthe sensor output signal so as to limit the error signal to the signalproduced by the radiation source alone, eliminating the effect of boththe static background radiation and the modulation signal from the erroroutput signal.

Another important feature of the control circuitry is an automatic gaincontrol circuit that maintains, by means of approximate feedbacksignals, a constant ratio between error output voltage and vehicledisplacement from the center of the guide line regardless of the levelof background illumination. In the present system, the guidance signalis detected by sensors at three separate positions, to the left andright sides of the guide line and directly above the guide line. Tomaintain gain control independent of the effect of background radiation,these sensors are positioned so that they each receive approximately thesame amount of background radiation when the sensing mechanism iscentered over the guide line, and the gain control circuit maintains aconstant difference between the output signal from the center sensor andeither the left or right sensors (or preferably the average signal fromthe left and right sensors) when the sensing mechanism is centered overthe guide line. The difference signal eliminates the effect ofbackground radiation and produces a signal representative of theposition of the guide line with respect to the sensing positions. Thecenter position signal can be modified by adding to it the absolutevalue of the difference between the left and right sensor signals, inorder to maintain a relatively constant output gain even when there issubstantial variations of the sensing mechanism from its centeredposition over the guide line.

Still another important aspect of the present invention is theemployment of improved line detection circuitry for preventing operationof the vehicle in the absence of a valid guide line. The existance of apositive difference in the output signals between the center sensor andthe left and right sensors in the presence of a valid line serves as thebasis for the line detection circuits. A separate line detection circuitcan be employed for detecting this difference or the feedback signalfrom the gain control circuit can be employed for detecting a validline. Appropriate circuits can also be included to compensate for linebrightness and to limit a valid line indication to situations where theguide line is no more than a predetermined distance from being centered.

The control mechanism also includes a filter network for limitingguidance response to radiation emitted by the guide line; and automaticstop circuitry for stopping the vehicle whenever the vehicle comes incontact with an obstacle in the path.

Each of the foregoing features is an important aspect of the presntinvention and provides substantial benefits in improving the reliabilityand line tracking ability of the present invention. These featuresminimize adverse effects caused by variations in backgroundillumination, line brightness, contrast between guide line andbackground surfaces, variations in stimulation radiation levels, andother factors which may cause improper performance of an automaticallycontrolled vehicle.

The control circuit of the present invention can be used advantageouslyin almost any type of automatic vehicle guidance system for causing avehicle to follow a guide line capable of emitting or reflecting aguidance signal. However, the present invention is particularly welladapted for use in connection with an invisible line following guidancesystem designed for industrial vehicles that are used in delivery orfloor maintenance applications, such as floor scrubbers, floor sweepers,vacuum cleaners, and the like. The present invention provides for thefirst time control circuitry for an automatic guidance mechanism that ishighly reliable and is well suited for floor maintenance applications,thus permitting considerable cost savings in floor maintenance expense.With an automatic floor maintenance machine employing the trackingsystem of the present invention, a floor scrubber can be actuated bypressing a single button at the entranceway to a large convention hall,and the scrubber will automatically follow a predetermined yet invisiblepath back and forth across the entire convention hall floor and thenturn itself off at the end of the job without the intervention of anoperator for the entire process. The machine can be programmed to turnsharp corners, go around any obstacles, or take any other course or paththat may be desired. The automatic stop circuitry automatically stopsthe vehicle when it comes in contact with physical obstruction (e.g., awastebasket) placed in its way as it moves along the guide line.

These and other advantages and features of the present invention willhereinafter appear. For purposes of illustration, but not of limitation,a preferred embodiment of the present invention is described below andshown in the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a vehicle, an automated floor scrubbingmachine, embodying the guidance system of the present invention.

FIG. 2 is a schematic plan view of the floor scrubber shown in FIG. 1,showing the guidance system of the present invention in block form.

FIG. 3 is a schematic diagram showing the relative positions of theexcitation lamp and photocell sensors employed in the guidance system ofthe present invention.

FIG. 4 is a schematic block diagram showing the interaction between thesensor circuitry of the present invention and the servo-operatedsteering mechanism of the vehicle.

FIG. 4a is a schematic block diagram showing the interconnection betweenthe sensor circuitry and the steering control mechanism in a rearsteering three-wheeled vehicle.

FIG. 5 is a block diagram showing a first embodiment of the sensor andline detection circuitry of the present invention.

FIG. 6 is a block diagram showing a second embodiment of the sensor andline detection circuitry of the present invention.

FIG. 7 is a schematic graph showing amplifier output wave forms atvarious points in the sensor circuitry.

FIG. 8 is a schematic graph showing sensor output voltage as a functionof the displacement of the guide line away from a position directlybelow the center sensor.

FIG. 9 is a schematic block diagram showing the guidance and controlcircuitry of an automatic floor scrubber operated by the vehicleguidance system of the present invention.

FIG. 10 is a schematic circuit diagram of the power supply and lightingcircuits of the present invention.

FIG. 11 is a schematic circuit diagram of the light detecting circuit ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

Referring now to the drawings, a vehicle 10 in the nature of a floormaintenance machine designed for scrubbing and cleaning a floor is shownin FIG. 1. The principles of the guidance system of the presentinvention can be employed with any type of vehicle that is adapted to besteered along a path on a surface, but for exemplary purposes, thedescription herein will refer only to floor maintenance machinery, moreparticularly to a floor scrubber.

A schematic plan view of floor scrubber 10, showing the variouscomponents of the floor scrubber and an automatic vehicle guidancesystem 12 constructed in accordance with the present invention, is shownin FIG. 2. The various components of the floor scrubber itself areconventional and are shown in block form for illustration purposes only.

The conventional aspects of floor scrubber 10 include a frame 14 thatrests on a tricycle running gear comprising a pair of parallel, spacedrear wheels 16 and a pivotable front wheel 18. Each of these wheels maycomprise a pair of closely spaced wheels (as shown in the drawing)instead of a single wheel. The vehicle is driven by means of a drivemotor 20 that drives wheel 18 by a suitable chain drive mechanism or thelike. Manual steering of the vehicle may be effected by an operatorriding the vehicle who pivots wheel 18 about its vertical axis by meansof a steering wheel and suitable gears (not shown).

For purposes of cleaning floors, floor scrubber 10 includes a solutionfeed mechanism 28 that feeds a detergent-containing solution to thefloor, a brush mechanism 30 that scrubs the floor with the solution, asqueegee 32 that wipes the cleaning solution from the floor after thebrushes have scrubbed the floor, and a vacuum device 34 that sucks thedirty water from the floor and transports it to a suitable holdingcontainer.

Guide Line

In order to provide a suitable path for the automatic guidance system 12of floor scrubber 10, a predetermined path preferably about an inch wideis described on a floor area 17 by means of a guide line 36. Because thepresent system does not operate from reflected radiation, guide line 36need not be formed of a material that forms a sharp visual contrastbetween the guide line and the surface of the floor to which the guideline is applied. In fact, guide line 36 desirably is virtually invisibleunder ambient lighting conditions. To provide a means for distinguishingbetween the guide line and the floor, guide line 36 includes a suitableamount of fluorescent material so that when the guide line is irradiatedby ultraviolet light, the fluorescent material fluoresces so as to emitvisible light in a predetermined frequency range. The preferredfluorescent material radiates visible light in the blue-green spectrum(approximately 450-500 nanometers) when stimulated by invisibleultraviolet radiation at about 360 nanometers. The visible light is thendetected by the guidance system and effective controls are provided tosteer the vehicle along the guide line.

Guidance Mechanism

The automatic guidance mechanism 12 employed in floor scrubber 10comprises a position sensing unit 13 mounted by means of a suitableframe 38 or the like to the pivotal mounting mechanism for wheel 18.Position sensing unit 13 is pivotally movable along with wheel 18 sothat if the unit is maintained over the center of the guide line, wheel18 is continuously placed in a direction such that the vehicle willfollow the guide line.

Position sensing unit 13 is adapted to sense the position of theguidance mechanism with respect to the guide line and generate an errorsignal proportional to the variation of the position of the sensing unitwith respect to a predetermined lateral position relative to the guideline. An error output voltage is generated by the sensing unit that isproportional to the displacement distance of the guide line away fromthe center of the sensing unit.

The error output signal produced by position sensing unit 13 istransmitted by means of lead 40 to automatic control apparatus 15 thatsteers the vehicle along the guide line in response to the error outputsignal. Automatic control apparatus 15 includes a servo-amplifier 42that amplifies the signal from lead 40, and the servo-amplifier drives areversible steering motor 22 to control the steering of the vehicle.Servo-amplifier 42 is connected to steering motor 22 by means ofseparate leads 44 and 46 for forward and reverse operation of steeringmotor 22. The steering motor is connected to pivotable wheel 18 to steerthe vehicle by means of a suitable sprocket 24 and drive chain 26. Othertypes of drive trains could also be employed.

A schematic representation of the manner in which guidance mechanism 12effects the steering of vehicle 10 is shown in FIG. 4. In that figure,position sensing unit 13 is shown in position directly over guide line36. Lead 40 is connected to servo-amplifier 42 and servo-amplifier 42 isconnected by means of leads 44 and 46 to drive reversible motor 22.Dotted line 48 extends between position sensing unit 13 and motor 22 toshow schematically that the steering action effected by the servo-motormaintains the position sensing unit over the center of the guide line.

Dotted line 48 represents an important feature of the present invention,namely, the mechanical feedback provided by the control mechanism toposition sensing unit 13. In prior automatic guidance systems, theposition sensing apparatus has been mounted in a fixed position on thevehicle, so that the vehicle itself must return to a centered positionover the guide line before a "no error" signal is produced by thesensing unit. The problem with this type of system is that the timeresponse characteristics of the system are undesirably slow, and thisimpairs the ability of a vehicle to quickly and accurately trace acomplex guide line.

In accordance with the present invention, mechanical feedback isprovided not by the actual movement of the vehicle itself, but by themovement of pivotable steering wheel 18 to a proper steering position.An error signal generated by position sensing unit 13 causes wheel 18 topivot so that the position sensing unit is maintained over the guideline, thus causing the wheel to continuously follow the position sensingunit along the guide line. By providing the feedback interconnection inthe vehicle through the steering mechanism as opposed to vehicleposition, the time response characteristics and, hence, themaneuverability and tracking ability of the vehicle are markedlyimproved.

In the exemplary embodiment described herein, a three wheeled vehiclewith a single, front steering wheel 18 is disclosed. The same principlesof operation would be employed in a vehicle having two front steeringwheels, except that the position sensing unit could be suspended fortransverse movement with the turning of the wheels by mechanical linkageinterconnected between the mounting mechanisms for the two wheels.

In a rear steering vehicle, such as a vehicle having a tricycle runninggear with a single pivotable wheel mounted in the rear and a pair offixed wheels mounted in the front, the same basic principles apply, butadditional mechanical linkage or gearing is necessary in order tocompensate for the fact that a rear steering wheel is turned in adirection opposite to the desired direction of the vehicle. One possibleinterconnection is shown schematically in FIG. 4a, wherein the guidancesystem is shown in a rear steering three wheeled vehicle having frontwheels 16' and a pivotable rear wheel 18'. Rear wheel 18' is connectedto a gear 19, and gear 19 is drivingly connected to gear 21. Gear 21 isconnected by a frame 23 to sensing unit 13'. When sensing unit 13'becomes displaced from guide line 36, an error displacement signalpivots rear wheel 18' and gear 19 in one direction (e.g.,counterclockwise), and this moves gear 21 and sensing unit 13' in theopposite direction (e.g., clockwise) until the sensing unit is againproperly positioned over the guide line. Other types of mechanicallinkages could be devised to accomplish the same purpose.

Considering the structure of position sensing unit 13 in more detail asshown in FIG. 2, this unit includes a radiation source 50 adapted tomake guide line 36 fluoresce at a point immediately below the sensingunit and a sensor mechanism adapted to detect the visible radiationemitted from the guide line and produce an output signal proportional tothe variation of the position of the sensing unit with regard to theguide line.

Radiation Source

Radiation source 50 comprises a conventional ultraviolet lamp that emitsradiation principally in the ultraviolet frequency spectrum. Pureultraviolet radiation is invisible to the eye, yet it causes fluorescentmaterial to fluoresce and emit radiation of a frequency that is visibleto the eye. As stated above, in the preferred embodiment of the presentinvention, the fluorescent material selected for use in connection withthe guide line is the type of material that fluoresces in the blue-greenregion of the visible spectrum (approximately 450-500 nanometers). Ifultraviolet lamp 50 radiates some visible light in the blue spectrum inaddition to the invisible ultraviolet light, suitable filters 52 (FIG.3) are employed in order to prevent the blue light from being reflectedoff of the floor and into the sensor means. Therefore, lamp 50 directsonly invisible radiation onto the floor and only invisible radiation isreflected from the floor. The only visible radiation produced as aresult of the radiation emitted from lamp 50 is fluorescent radiationemitted from fluorescent guide line 36.

This feature of the present invention provides an important advantageover prior art reflected radiation tracing systems, because in reflectedradiation systems, the same radiation is reflected from both thebackground and the guide line being traced, and the radiation reflectedfrom the background minimizes the distinctiveness and the contrastbetween the guide line and the background surface. This distinction isparticularly troublesome when a bright background surface is employed.When a fluorescent system is used, the fluorescent material can bestimulated with invisible light of a second frequency range and thefluorescent material will in turn emit a visible light of a firstfrequency range. The sensor mechanism can be made selectively responsiveto the particular frequency range of visible light emitted by the guideline, such that the sensor is responsive only to the visible radiationof the fluorescent guide line and is not responsive to invisibleradiation reflected off the background surface. The contrast between theguide line and the background surface is therefore maximized, therebyproviding a marked improvement in reliability over reflected lighttracing systems.

Sensor Circuitry

Considering the structure of the sensor circuits in more detail, thesensing unit includes a left photocell 56, a right photocell 58, and acenterline photocell 60 mounted over the guide line. The system isdesigned so that centerline photocell 60 is positioned directly overguide line 36 and the left and right photocells are positionedequidistant on each side of the guide line. Preferably, the left andright photocells are each spaced about two inches from the centerlinephotocell and are inclined at an angle of about 15°-18° from ahorizontal position. This insures a broad and continuous viewing areafor tracking the guide line and provides additional advantages which arediscussed below in connection with the AGC feedback circuits. Otherpositions and viewing angles could work satisfactorily, subject to therelative position requirements discussed in connection with the AGCfeedback circuit.

The photocells employed in the preferred practice of the presentinvention are photoconductors having a relatively slow time constant andhaving a peak response frequency range that conforms with or at leastoverlaps the fluorescent guide line. Such photocells typically areslightly responsive to ultraviolet radiation, so filters 61 are employedto filter out ultraviolet radiation reflected from the backgroundsurface. Desirably, the photocells are selected so as to have a peakresponse in the frequency range of the light emitted by the fluorescingguide line, in order to enhance the line distinguishing abilities of thesystem.

In a first embodiment of the sensor circuitry shown in FIG. 5, thephotocell signals are sensed as voltage signals across gain adjustresistors 55, 57, and 59 (which provide a means for compensating forgain variation in the photocells), with the voltage signalscorresponding to the intensity of the visible light received from thefloor. The photocell signals pass first through D.C. blocking capacitorsand then are amplified through preamplifiers A1, A2, and A3,respectively. The respective signals then pass through signaldemodulation and sample and hold circuits to eliminate the effect ofstatic background illumination, and the remaining signals appear asphotocell output signals in leads 62, 64, and 66 for the right, left,and centerline photocells, respectively. The photocell output signalsfor the right and left photocells are then subtracted in differentialamplifier A8. The output of amplifier A8 passes through gain changeamplifier A15 (which is discussed below), and this output serves as theerror displacement output signal for operation of the steeringservo-amplifier.

The basic differential circuit of FIG. 5 also is employed in the secondembodiment of the sensor circuit shown in FIG. 6. For convenience,identical elements are given the same reference numbers in bothembodiments.

Modulation Circuits

In order to maximize the effectiveness and reliability of the controlapparatus of the present invention, it is important to be able toeliminate all effects of static ambient illumination from the photocelloutput signals appearing at leads 62, 64, and 66. In order to accomplishthis result, the ultraviolet lamp is flashed on and off by power supplyand lighting circuits 70 at a predetermined synchronized frequency rate.The photocell outputs are then passed through synchronized demodulators72, 73, and 74, which strip the lamp induced signals from the backgroundsignal and pass the lamp signals through sample and hold amplifiers A4,A5, and A6 to photocell output leads 62, 64, and 66, respectively.

The operation of this modulation circuitry is shown in FIG. 7 whichshows the output wave forms at various points in the circuit as afunction of time. In FIG. 7a, at the top of this graph, the wave formcreated by the flashing ultraviolet lamp is shown as a square wave. Inthe preferred embodiment of the present invention, the frequency of theflashing ultraviolet light is selected to be 144 Hz. This frequency isnot critical but is selected at this value to minimize the harmonicoverlap between the flashing of the ultraviolet light and the 120 Hzlight component of conventional alternating current fluorescent lighting(which appears in background illumination).

Synchronized demodulators 72, 73, and 74 in conjunction with sample andhold amplifiers A4, A5, and A6 effectively measure the differencebetween the light levels during the time when the ultraviolet light ison and off, thus providing a measure of the increased light due to thefluorescene of the guide line. These differential signals appearing atthe outputs of amplifiers A4, A5, and A6 (herein referred to as thephotocell output signals) do not include the component of staticbackground illumination. Without a modulation circuit of the typedescribed, variations in the intensity of static backgroundillumination, such as would occur in operating the system under varyinglighting conditions or in tracing a line on a variable black and whitecheckered tile floor, could cause serious variation in the error outputsignal produced by the guidance system. This in turn would affect thestability and the accuracy of the steering mechanism.

The actual operation of the synchronized demodulators and the sample andhold circuits can best be described with reference to FIGS. 7b, 7c, and7d. FIG. 7b shows the decrease in the output voltage of preamplifiersA1, A2, and A3 as a function of time. When the ultraviolet light isflashed on and the guide line is caused to fluoresce, the photocells(which are photoconductors in the present invention) begin to decreasein resistance in accordance with the time constant of the photocells.This gradual decrease in resistance is shown at the output of thepreamplifiers as a decreasing voltage level. The photocell timeconstants are slow enough so that the preamplifier outputs appear asgradually decreasing voltages.

As shown in FIGS. 5, 6, 7c, the power supply and lighting circuits 70generate a series of strobe or synch pulses in synchronization with theflashing frequency of the lamp driver, with a first synch pulse beingtransmitted to the demodulators just after the start of each lampflashing cycle by a lead 69 and a second synch pulse being transmittedto the demodulators just before the end of each lamp flashing cycle by alead 71. The circuit details of power supply and lighting circuits 70are shown in FIG. 10. The primary system operating voltage (36 VoltsD.C.) appears across terminals 91 and is converted to 16 kHz A.C. by a16 kHz inverter 93 and power transformer 95. A diode bridge and filtercapacitors 97 rectify the A.C. voltage on the secondary of the powertransformer, which then provides the pulse and minus voltage supplies tooperate the automatic system.

A high voltage winding 99 on the power transformer, in conjunction witha ballast 101, provides the proper conditions to illuminate theultraviolet light 50 at 16 kHz. Relay 103 and associated drivetransistor Q1 short the light for 7 seconds after power is applied inorder to heat the light filaments and allow proper operation when relay103 becomes de-energized.

A transistor Q2 and an associated diode bridge 105 and pulse transformer107 short out the lamp at a 144 Hz rate, as determined by a symetricalastable multivibrator 109 and a transistor Q3. This flashes the lampon-and-off at 144 Hz.

A string of three monostable multivibrators, 111, 113, and 115 (oneshots) also are driven by the astable multivibrator and these elementsprovide the strobe or synch pulses to the synchronous demodulators. Afirst 0.4 ms pulse produced at the output of transistors Q4 by one-shot111, initializes the demodulator when the lamps turns on, and a second0.4 ms strobe pulse produced at the output of transistor Q5 transfersthe sensor photocell data to the sample and hold amplifiers 3.0 mslater, just before the lamp turns off.

Each synchronized demodulator comprises a series capacitor 75 connectedto a grounded field effect transistor ("FET") 77 and then to a seriesconnected FET 79. FET 77 and FET 79 are both effectively normally openand become closed circuits upon receipt of synch pulses from leads 69and 71, respectively. Each synchronized demodulator is connected to agrounded capacitor 81 and then to the input of its respective sample andhold amplifier.

The operation of the synchronized demodulation circuit is as follows:When both FETs are open, no signal is transmitted. When a first synchpulse is generated FET 77 is closed for the duration of the pulse (0.4ms). This causes the output voltage of the photocell preamplifier to beimpressed on capacitor 75 and held. When the second synch pulse isgenerated (3.0 ms after the start of the first pulse), FET 79 is closed,and this causes the difference between the voltage across capacitor 75(i.e., the voltage at the time of the first synch pulse) and the voltageat the time of the second synch pulse to be registered across capacitor81. This difference voltage is the input voltage for the sample and holdamplifier. This voltage is reflected at the output of the sample andhold amplifier as a constant D.C. voltage (FIG. 7d), with the D.C.voltage changing only when a subsequent pair of synch pulses cause a newdifferential voltage to be impressed on capacitor 81.

Although the flashing of the ultraviolet lamp is effective to eliminatevirtually all static background illumination, a slight response may bepassed through the synchronized demodulators as a result of the beatfrequency between the 120Hz fluorescent room lighting and the 144 Hzvisible radiation emitted by the fluorescent line (24 Hz component).This is shown in FIG. 7e. The magnitude of this component, however, isrelatively small compared to the amount of light effectively blocked outby the modulation circuitry. Further, the minor effect due to the 24 Hzcomponent as a result of the fluorescent room lighting generally isbalanced between the left and right photocells, so that the differentialamplifier will remove this component from the error output voltage (asshown in FIG. 7f).

The same modulation circuitry is employed in the second embodiment ofthe sensor circuitry of the present invention shown in FIG. 6. Identicalcomponents employ the same reference numbers in both embodiments.

Agc feedback Circuits

In addition to eliminating the effect of static background illuminationby the use of modulation circuitry, additional feedback circuitry isemployed in order to further enhance the stability of the system bymaintaining a constant ratio between error output voltage versusdisplacement from the center of the guide line. To accomplish thiseffect an automatic gain control circuit ("AGC") is incorporated in thecircuitry.

In the first embodiment of the present invention shown in FIG. 5, theAGC is designed to maintain a constant summation of the left and rightphotocell output signals. In order to accomplish this effect, the outputvoltage of the left and right photocells is fed back through a summationamplifier A9 and the sum of the right and left output signals iscompared with a reference voltage in AGC amplifier A7, which is a highgain integrator. The difference between the summation voltage and thereference voltage is fed back through line 80 to the photocell input.This same differential input is fed by lead 82 through level detectoramplifier A14 and then by lead 83 to FET 85 in amplifier A15 in order toincrease output gain for very dim lines. In a situation where the AGClevel exceeds 2.5 volts (i.e., a very dim line) amplifier A15 increasesthe output gain by a factor of 2 to help maintain a constant deviationgain.

The drawback with this type of circuit, however, is that when the sum ofthe left and right photocell outputs is maintained at a constant level,the closed loop signal gain varies with the level of fluorescentbackground illumination. When fluorescent background illumination ishigh, the feedback circuit lowers the gain, even though there may be nosignal variation with respect to signal radiation. This alters theclosed loop gain and adversely affects system stability.

In the context of the present invention, even with the benefit of afluorescent guide line and modulation circuitry, varying fluorescentbackground illumination causes undesirable variation in the output gainin the system. In order to eliminate the effect of fluorescentbackground illumination in the signal feedback circuit, the improvedfeedback circuit shown in FIG. 6 was developed.

This improved feedback circuit maintains a constant difference betweenthe centerline voltage (as modified herein) and the average of the leftand right photocell outputs. When the positions of the photocells areadjusted so that they receive approximately the same amount ofbackground illumination from the same viewing area, the differencefunction eliminates the background illumination component of the totalsignal and leaves a reference voltage that is dependent only on guideline signal strength.

In the improved feedback circuit, the average of the left and rightphotocell output signals is produced in summation amplifier A16 and isintroduced into a differential amplifier A17. To produce the other inputsignal to differential amplifier A17, amplifiers A18 and A19 compute theabsolute value of the error output signal (i.e., the difference betweenthe right and left photocell signals), and this value is added to theoutput voltage of the centerline photocell in amplifier 24. Thissummation signal is introduced into amplifier A17 and the differencebetween this signal and the average output signal of the left and rightphotocells is compared with a reference voltage in AGC amplifier A20,which again is a high gain integrator. The difference is the AGC voltagesignal, and this signal is fed back to the photocell inputs to maintaina constant closed loop signal gain in the system.

The significance of the improved AGC circuitry in the second embodimentof the present invention is shown in the graphs set forth in FIG. 8. Inthat figure, the left, right, and centerline photocell output signalsare shown as functions of displacement of the guide line from a positionimmediately below the centerline photocell. Line 86 shows the average ofthe left and right photocell signals, and line 88 shows the error outputsignal (i.e., the difference between the left and right photocellsignals). The sum of the centerline photocell output signal and theerror output signal 88 is shown by line 90. The difference between lines90 and 86 represents the AGC voltage that is maintained as a constant inthe present invention.

In order to maintain a constant closed loop gain with the improved AGCcircuit without reference to background illumination, it is necessarythat the centerline photocell be positioned such that the backgroundillumination received by it is effectively the same as the backgroundillumination received by the left and right photocells when the guideline is centered. Since the intensity of illumination received by aphotocell is a function of the photocell viewing area from which theillumination is received and the effective distance through which theillumination passes before it is received by the photocell, thebackground illumination received by each photocell with the guide lineimmediately below the centerline photocell can be made equal by properpositioning of the photocells and, if necessary, by placing appropriateshields on the photocells. In the preferred embodiment of the presentinvention, the left and right photocells are each positioned about twoinches away from the centerline photocell and the left and rightphotocells are inclined at an angle of about 15°-18° with respect to thehorizontal. Some additional screening is necessary in order to achievethe desired effect of making the fields of view of the three sensorsequal with respect to the background area when the guide line ispositioned immediately below the center sensor.

With the photocells balanced in this manner, the difference betweencenterline photocell output and the average of the side photocellseliminates the effect of background illumination from the feedbacksignal when the guide line is approximately centered.

Although the use of a feedback signal comprising the difference betweencenterline output and the average of the side outputs is effective tomaintain a constant closed loop signal gain without interference frombackground illumination, it can be observed from FIG. 8 that the signalgain will remain constant only as long as the guide line isapproximately centered and the effective background illuminationreceived by the photocells is equal. When the guide line is displaced toeither side of center, the effective amount of background illuminationreceived by each photocell is changed, thus permitting a component ofbackground illumination to enter the feedback signal. Also, thedifference between centerline voltage and the average of the sidephotocell output becomes smaller. The error introduced by displacementof the guide line can be avoided at least partially by making the timeconstant of the AGC circuit sufficiently slow and the reaction time andaccuracy of the servo-control sufficiently high so that the vehicle willbe returned to a zero error position before the AGC circuit reacts tochange the feedback signal because of displacement.

In order to avoid closed loop gain variance due to the decreasingdifference between centerline output and the average side photocelloutput with increasing displacements away from the centerline, theabsolute value of the error output signal can be added to the centerlinevoltage before subtracting the average of the left and right photocells.As shown in FIG. 8, by modifying the centerline signal in this manner,the AGC voltage can remain relatively constant even for substantialdisplacements of the guide line away from the centerline.

To summarize the object and effect of the improved AGC feedback signalon the present invention, if the centerline photocell is positioned sothat it receives approximately the same background viewing illuminationas the photocells on each side of it, and the centerline photocelloutput (as adjusted by adding to it the sum of the absolute value of thedifference between the right and left photocells) is subtracted from theaverage value of the left and right photocell outputs, the differentialis a relatively constant voltage that is independent of the level ofbackground illumination present.

The importance of maintaining a relatively constant ratio between erroroutput voltage and displacement is that the stability of the vehiclesteering system is dependent upon the maintenance of the gain of thesystem at a relatively constant level in order to provide a quicksteering response to directional variation. It is necessary that thegain of the servo-amplifier be maintained at a high level, yet thislevel cannot be too high or the steering response would be too quick andthe system will oscillate. Oscillation might be observed in this systemby the continuous wobbling or fluctuation of the vehicle back and forthacross the guide line instead of stabilizing on the center of the guideline. On the other hand, if the gain is too low, the steering responsemay be too slow to bring the vehicle back into alignment with the guideline at all on sharp turns, with the result being that the vehicle maywander completely away from the guide line. To maintain the closed loopgain and the response of the servo-system at the proper level, it isnecessary that the level of signal output versus displacement bemaintained at a constant level.

The principal purpose of the feedback signal is to prevent variations inline brightness or contrast or fluctuation in the level of ultravioletradiation from affecting output gain. Thus, when signal level drops, theAGC boosts gain and when the signal level rises, the AGC lowers gain. Bymaking the feedback circuit independent of background illlumination,gain control becomes responsive (as it should) only to changingconditions in the displacement error signals computed with the photocelloutput in response to guide line variations.

Line Detection Circuitry

Another important feature of the present invention is the incorporationof line detection circuitry to prevent the system from operating unlessthe vehicle and the vehicle guidance mechanism are placed over a validguide line. If the system did not incorporate line detection circuitry,the automatic guidance system could operate the vehicle even if thevehicle were not on a valid guide line. Thus, there would be nodirectional control to the vehicle at all and the vehicle could wanderaway in any direction.

A first embodiment of a line detection system is shown in FIG. 5. Inthat embodiment a valid line is first indicated when the centerlinephotocell output is greater than both the left and right photocelloutputs. In order to achieve this effect, the difference between thecenterline output and the left output is computed in amplifier A10 andthe difference between the centerline photocell output and the rightphotocell output is computed in amplifier A12. These difference voltagesare applied to level detector amplifiers A11 and A13, wherein they arecompared with a suitable reference voltage. To initially capture a line,the centerline output must be greater than both the left and rightchannels and the signal strength must be of sufficient strength towarrant a capture. This condition is transmitted to AND gate 102, whichactuates a flip-flop 104 to produce a valid line output signal atterminal 106. This assures that the programmed line is bright enough andof proper width. This also prevents false line indication such as mightbe produced by a piece of paper or a black and white checkerboard tilepattern. Once the line has been captured, the logic reverts to therequirement that the centerline output be greater than either the leftor right channel, as indicated by NOR gate 108. If this condition is notmet, NOR gate 108 causes flip-flop 104 to indicate an invalid line.

The purpose of having separate logic conditions for initially capturinga line and indicating a valid line once the line has been captured is toincrease the effective field of view once the line has been captured.This prevents the generation of a no-line indication in situations suchas when the vehicle is traveling at a high speed and turning a sharpradius on a dim line.

As stated above, the difference between the centerline output and theleft and right outputs are compared in level detectors A11 and A13 to asuitable reference voltage. This voltage, in ordinary circumstances,could be any predetermined voltage level which is chosen to satisfy thecondition that the centerline output signal be sufficiently greater thanthe left and right output signals to warrant a positive output signal.However, on extremely bright lines, the difference between thecenterline output and the output of the side sensors decreases becauseof the nonlinear response of the photocells. Under these conditions itis desirable to reduce the level detector trip point. Thus, thereference voltage employed is the AGC output signal through an amplifierA21 and an associated limiter 107. These components effectively reducethe trip points for extremely bright lines.

Because of the different viewing angles of the center photocell withrespect to the left and right photocells, the difference outputs fromamplifiers A10 and A12 contain beat frequency interference from the 120Hz room fluorescent lighting. To detect dim programmed lines, leveldetectors A11 and A13 must be set to detect small brightnessdifferences. To prevent false line indications because of the 24 Hz beatfrequency output caused by the 120 Hz fluorescent fixtures, resetable120 ms time delays 112 and 114 are used at the output of the leveldetectors to assure that the outputs remain valid for at least 120milliseconds.

A second embodiment of a line detector system is shown in FIG. 6. Inthis embodiment, the AGC output signal is employed for the purpose ofdetermining a valid line. It can be observed from the nature of the AGCfunction (i.e., to maintain a constant voltage differential between thecenterline output voltage and the average output voltage of the sidephotocells) that if the AGC amplifier is working in its permissibledynamic range, a valid line must exist, because the difference betweenthe centerline output and the side sensors can be maintained at apredetermined constant. Therefore, the presence of a valid line isdetected by sensing the AGC voltage. If, on the other hand, there is novalid line to be detected, the AGC amplifier cannot maintain a constantvoltage differential, and the amplifier rapidly becomes saturated.

In order to employ the AGC voltage as a valid line indicator, the outputof the AGC voltage is connected to a level detector amplifier A22 and iscompared therein to a reference voltage. The voltage is adjusted so thatif the AGC amplifier output approaches saturation, level detectoramplifier emits a negative line indication.

This negative line indication could itself be used as a control signalfor controlling the operation of the automatic line follower.Alternatively, this output can be connected to an AND gate 109 alongwith appropriate circuitry for limiting the permissible displacementrange for initial capture of a guide line. In the preferred system ofthe present invention, without additional limiting, the electronics candetect a valid line as far as 21/2 inches away from the desiredcenterline. This can cause a dramatic capturing reaction when theautomatic control device initially engages the system. To prevent thisfrom occurring, a displacement limiting device is incorporated into theline detection circuitry. This displacement limiting device employs theabsolute difference between the right and left photocell outputs andcompares this difference with a suitable reference voltage (e.g., onevolt) in level detector amplifier A23. Whenever the difference betweenthe right and left photocell outputs exceeds this predeterminedreference voltage, a negative line signal is transmitted to AND gate 109and a no-line output signal is produced. In the preferred practice ofthe present invention, the reference voltage for amplifier A23 isadjusted so that the displacement of the tracking mechanism must exceedabout one inch on either side of the guide line before an invalid linewill be indicated.

Automatic Scrubber Operation

In FIG. 9, a block diagram of the entire electrical circuitry of theautomatic scrubber 10 is shown. Starting with a power source 118 shownin the upper right hand corner in this drawing, since this is a remotecontrol, indoor, industrial vehicle, the power source is a series of DCbatteries 120 connected in series in order to produce a 36-volt powersupply. A master switch 122 in series with the batteries controls thepower supply to all elements of this system.

Starting at the left hand side of the drawing in describing the variouselements of the system, lamp 50 is driven by means of power supply andlighting circuits 70, shown schematically in block form in FIG. 9 andshown in detail in FIG. 10. A lamp photocell 126 is employed in additionto the other photocells in order to indicate whether the lamp is on. Ifthe lamp is not on, a light relay 128 connected to power supply 70 bylead 125 breaks the circuit and prevents the system from operating.

The lamp detection circuit is shown as a portion of the schematic blockdiagram of the power supply and lighting circuits 70 in FIG. 9, and thedetails of this circuit are shown in FIG. 11. As shown in that FIG. 11,lamp photocell 126 is connected through transistors Q6 and Q7 to producea logic signal output at terminal 127, wherein the transistor Q7 isturned off when there is no light and is turned on when the light is on.

Photocells 56, 58, and 60 comprising the guide line sensing mechanism ofthe present invention are connected to the sensor and line detectorcircuits which are shown in block form as element 130. The displacementerror output signal (i.e., the output of differential amplifier A8 inFIGS. 5 and 6) is connected to servo-power amplifier 42 through a servocompensation amplifier 131, and this servo power amplifier drivessteering motor 22 in either direction. The output of the line detectionsystem, as indicated in FIGS. 5 or 6 to be the "line logic output" isfed through a 300 millisecond time delay 132 to a line relay switch 134which operates contacts 134a in line 135. Line relay switch 134 breaksthe circuit connection in line 135 if an invalid line is indicated. The300 millisecond time delay prevents minor breaks in the line or minorvoltage fluctuations from indicating a no-line situation.

Relay switches 134a and 128a are connected in series with an obstaclerelay 136. Obstacle relay 136 is deactuated by either stop switch 138 orby a bumper switch 140. Stop switch 138 is a manually operated stopswitch which can be actuated by an operator in an emergency or similarsituation. Bumper switch 140 is an electrical contact switch on thebumper or outer perimeter of the vehicle, which is actuated whenever thevehicle strikes an obstacle. Bumper switch 140 typically could be formedof a substance called "touch-tape", which is a tape material that can beapplied to the exterior of a vehicle and which makes a contact betweentwo terminals when pressure is applied to the tape. The purpose ofbumper switch 140 is to effect the automatic deactuation of the controlsystem and forward drive system whenever a vehicle comes in contact withan obstacle in the path of its guide line.

Line relay 134a and light relay contacts 128a are connected in serieswith obstacle relay contacts 136a, and these contacts are all connectedin series with an auto engage and latch relay 142, and an auto engagemomentary actuation switch 144, which is connected to a ground. When 36volts are applied to line 135 and the line relay, light relay, andobstacle relay switches are all closed, the automatic mode of theapparatus can be engaged by pressing the auto engage switch momentarily.This closes the contact and energizes auto engage and latch relay 142.The engagement of the relay 142 closes the associated relay switches142a and 142b. Since switch 142a is grounded, the release of auto engageswitch 144 does not break the circuit but leaves the coils in anenergized state. Closing contacts 142b impresses 36 volts along lead 146and this provides the operating voltage for the entire system.

Before contacts 142b can be closed, it is necessary first that themachine be placed in its automatic mode by means of mode selector switch150. Mode selector switch 150 has two positions, an automatic modeposition and a manual mode position (shown in FIG. 9). When in manualmode position, the operating voltage is connected through branch 152 toa brush motor 154, vacuum motor 156, and a solution feed valve 158, eachof which is independently actuatable by manual switches 160, 162, and164, respectively. A separate output lead 166 from mode selector switch150 leads to ground through a dynamic brake relay 168. When this relayswitch is actuated by moving the mode selector switch to its manualposition, contacts 168a move from a position wherein circuit 169 shortcircuits traverse motor 20 to a position wherein the motor may be drivenby the 36-volt power supply applied at terminal 172. The purpose ofproviding a short circuit for the traverse motor when the motor is to bedeactuated is that the short circuit causes the motor to act as agenerator and provides dynamic braking to bring the motor to a halt.

In order to complete the voltage connection between the power supplyapplied to terminal 172 and the traverse motor, it is necessary first todepress a foot pedal switch 174. The depression of this switch actuatesrelay 176 which closes contacts 176a, thus completing the circuit byconnecting terminal 178 to ground.

In order to reverse the direction of the vehicle when the vehicle is inits manual mode, a foot pedal reverse switch 180 is depressed, and thisactuates relay switch 182. This in turn switches the contacts 182a and182b of the relay switch to a position wherein the 36-volt power suppplyis applied to the traverse motor in an opposite direction.

When mode selector switch 150 is placed in automatic mode position, the36 volt source voltage is impressed upon the power supply and lightingcircuits 70. This will effect the closing of line relay switch 134 andlight relay switch 128 if a valid line is detected and if the light isnot burned out. At this point, auto engage switch 144 can be engagedmomentarily in order to close contacts 142a and 142b. This impresses 36volts in the circuit through lead 146. Once the auto engage switch hasbeen depressed, power to the servo power amp is provided through lead148, and voltage is applied to an audible alert signal 184 through atone generator 186 in order to audibly indicate that the machine is inoperation. Another safety device is a rotating beacon including a motor188 and light signal 190 in parallel. The rotating beacon also isactuated by depressing auto engage switch 144. Relay switch 176 andcontacts 176a are actuated without the depression of foot pedal switch174 by applying the voltage in line 146 to relay 176 through line 192.Similarly, the dynamic brake relay switch is actuated by applying theinput voltage to the brake relay through lead 194. Diodes 196 and 198are inserted in leads 192 and 194 respectively in order to isolate theseleads when the load selector switch is in its manual position.

When auto engage switch 144 is depressed, the 36-volt power supply issimilarly connected through diode 200 to lead 152 which powers the brushmotor, vacuum motor, and solution feed valve. Switches 160, 162, and 164still have to be manually actuated before these elements are placed intooperation.

The operation of the apparatus of the present invention should beapparent from the foregoing description. Briefly summarizing thisoperation, when the floor scrubber is to be actuated manually, theoperator places the mode selector switch in its manual position andmounts the floor scrubber. The traverse motor is actuated by depressingfoot pedal switch 174 and the brush motor, vacuum motor, and solutionfeed valve are actuated manually. Steering is effected manually and noneof the automatic systems are in operation. In order to switch the systemto automatic mode, the mode selector switch is moved to its automaticmode position, and the auto engage button is depressed. The operatordoes not have to depress foot pedal switch 174, but he still mustactuate the brush motor, vacuum motor, and solution feed valve switchesmanually. In either case, master switch 122 must be actuated before anypower can be applied to the system, whether the system is in its manualor automatic mode.

When the system is operating in its automatic mode, the system willautomatically shut down if a no-line indication is received by linerelay 134; if the light relay 128 indicates that the ultraviolet lamp isnot lit; if the operator depresses a stop switch 138, or if the vehiclecomes in contact with an obstruction. Suitable warning lights areemployed in the system in order to indicate the reason why the systemhas shut down, and when this condition has been rectified, the systemcan be reactuated by simply depressing the auto engage switch a secondtime.

It should be understood that the foregoing embodiments of the presentinvention are merely exemplary and the preferred practice of the presentinvention and that various changes and modifications may be made in thearrangements and details of construction of the embodiments describedherein without departing from the spirit and scope of the presentinvention.

The embodiments of the invention in which an exclusive property ofprivilege is claimed are defined as follows:
 1. In a vehicle guidancemeans for guiding a vehicle along a guide line, wherein guidance systemradiation means causes the guide line to emit or reflect a guidancesignal, sensor means senses the guidance signal and produces adifferential error output signal representative of the deviation of thevehicle position from the guide line, and control means automaticallycontrols the position of the vehicle in accordance with the error outputsignal, the improvement comprising:feedback means for maintaining agenerally constant ratio between error output signal strength anddisplacement of the guide line from a predetermined centerline position,regardless of the strength of background radiation, said feedback meansmaintaining said ratio generally constant, as long as the vehicle isfollowing the guide line and the guidance signal is stronger than thesignal produced by background radiation when the radiation means isswitched on; modulation means for modulating the guidance signal with apredetermined modulation signal, said modulation means modulating theguidance signal by switching the radiation means on and off atsynchronized intervals, the modulation means further generating firstand second synchronized pulse signals once each cycle, the firstsynchronized pulse being generated at a first time intervalrepresentative of the time when the radiation means is switched on andthe second synchronized pulse being generated at a second time intervalrepresentative of the time when the radiation means is switched off,each time interval being a relatively small portion of the total timethe radiation means is switched on and off each cycle; and demodulationcircuit means for demodulating the error output signal of the sensormeans so as to eliminate the effects of static background radiation andthe modulation signal from the error output singal, thus limiting theerror output signal to the output signal produced by the guidancesignal, said demodulation circuit means comprising synchronizeddemodulation means for measuring the level of the output signal of eachsensor means during each first and second time interval and measuringthe difference between said signals, said demodulation circuit furthercomprising sample and hold amplifier means for producing and holding aDC output signal at a level equal to said difference until the value ofthe difference signal changes in a subsequent cycle.
 2. Vehicle guidancemeans as claimed in claim 1 wherein:the change in value of the sensormeans output signal lags the change in the guidance signal in accordancewith the time constant of the sensor means; and the demodulation meansmeasures the difference between the sensor means output signals justafter the radiation means has been switched on and just before theradiation means is switched off.
 3. Vehicle guidance means as claimed inclaim 1 wherein:the modulation means generates a first synchronizedpulse at a selected time when the radiation means is on and a secondsynchronized pulse at a selected time when the radiation time is off;the synchronized demodulation means comprises a first capacitor meansconnected in series with the output of the sensor means, a groundedfirst field effect transistor means connected to the output of the firstcapacitor means, a series-connected second field effect transistor meansconnected to the output of the first capacitor means and a groundedsecond capacitor means connected to the output of the second fieldeffect transistor means, both field effect transistor means beingnormally non-conductive, the first field effect transistor means beingrendered temporarily conductive by one synchonized pulse and the secondfield effect transistor means being rendered temporarily conductive bythe other synchronized pulse; and the sample and hold amplifier means isconnected across the second capacitor means and produces a DC outputsignal representative of the magnitude of the signal across the secondcapacitor means.
 4. Vehicle guidance means for guiding a vehicle along aguide line comprising:guidance system radiation means for causing theguide line to emit or reflect a guidance signal; sensor means forsensing the guidance signal and producing a differential error outputsignal representative of the deviation of the vehicle position from theguide line, the sensor means producing separate output signalsrepresentative of guidance signal intensity at at least three differentpositions with respect to the guide line, said positions being on theleft and right sides of the guide line and a center position over theguide line when the guide line is centered with respect to the sensormeans, the sensor means being positioned in the vehicle such thatapproximately the same amount of background radiation is received at allthree positions when the guide line is centered with respect to thesensor means; control means for automatically controlling the positionof the vehicle in accordance with the error output signal; feedbackmeans for maintaining a generally constant ratio between error outputsignal strength and displacement of the guide line from a predeterminedcenterline position, regardless of the strength of background radiation,said feedback means maintaining said generally constant ratio as long asthe vehicle is following the guide line and the guidance signal isstronger than the signal produced by background radiation when theradiation means is switched on, said feedback means maintaining saidconstant ratio by maintaining a constant difference between the outputsignal from the center position and the output signal from the averageof the output signals from the other positions when the guide line iscentered with respect ot the sensor means, said feedback means includingmeans for modifying the output signal from the center position by addingto it the absolute value of the difference between the left and rightposition output signals, the difference between this modified centerposition output signal and the average of the left and right positionoutput signals being the difference signal maintained at a substantiallyconstant level by said feedback means.
 5. Vehicle guidance means asclaimed in claim 4 wherein:the radiation means is adapted to cause theguide line to emit or reflect a guidance signal in the form of radiationcapable of being sensed by a photocell; and the sensor means comprisesseparate left, right and center line photocells positioned respectivelyat the left, right and center positions with respect to the guide line.6. Vehicle guidance means for guiding a vehicle along a guide linecomprising:guidance system radiation means for causing the guide line toemit or reflect a guidance signal; sensor means for sensing the guidancesignal and producing a differential error output signal representativeof the deviation of the vehicle position from the guide line, the sensormeans being adapted to produce separate output signals representative ofguidance signal intensity at at least three different positions withrespect to the guide line, said positions being on the left and rightsides of the guide line and a center position over the guide line whenthe guide line is centered with respect to the sensor means, the sensormeans being positioned in the vehicle such that approximately the sameamount of background radiation is received at all three positions whenthe guide line is centered with respect to the sensor means; controlmeans for automatically controlling the position of the vehicle inaccordance with the error output signal; feedback means for maintaininga generally constant ratio between error output signal strength anddisplacement of the guide line from a predetermined centerline position,regardless of the strength of background radiation, said feedback meansmaintaining said generally constant ratio as long as the vehicle isfollowing the guide line and the guidance signal is stronger than thesignal produced by background radiation when the radiation means isswitched on, said feedback means maintaining said generally constantratio by producing a feedback signal to the senor means that maintains aconstant difference between the output signal from the center positionand the output signal from at lest one or the average of the otherpositions when the guide line is centered with respect to the sensormeans; and line detection means for preventing the operation of thevehicle guidance means whenever the feedback signal exceeds apredetermined level.
 7. Vehicle guidance means as claimed in claim 6wherein the line detection means is further adapted to prevent operationof the vehicle guidance means if the absolute difference between thesensor means output signals at the left and right positions exceeds apredetermined value.
 8. Vehicle guidance means for guiding a vehiclealong a guide line comprising:guidance system radiation means forcausing the guide line to emit or reflect a guidance signal; sensormeans for sensing the guidance signal and producing a differential erroroutput signal representative of the deviation of the vehicle positionfrom the guide line, the senor means being adapted to produce discreetoutput signals representative of guidance intensity at at least threepositions with respect to the guide line, two of said positions being tothe left and right of the guide line and the third position being suchthat a predetermined relationship exists between the output signalsrepresentative of the third position and at least one of the otherpositions when the vehicle guidance means is positioned over the guideline, the sensor means being positioned in the vehicle such thatapproximately the same amount of background radiation is received at allthree positions when the guide line is centered with respect to thesensor means; control means for automatically controlling the positionof the vehicle in accordance with the error output signal; feedbackmeans for maintaining a generally constant ratio between error outputsignal strength and displacement of the guide line from a predeterminedcenterline position, regardless of the strength of background radiation,said feedback means maintaining said generally constant ratio as long asthe vehicle is following the guide line and the guidance signal isstronger than the signal produced by background radiation when theradiation means is switched on, said feedback means maintaining saidgenerally constant ratio by maintaining a predetermined relationshipbetween the sensor means output from the third position and at least oneor a combination of the other positions at least when the guide line iscentered with respect to the sensor means, said feedback means receivingan error signal from the sensor means that is representative of theexisting relationship between the sensor means output and generating afeedback signal to the sensor means to correct the relationship to theextent necessary to maintain said predetermined relationship, at leastone of said error and feedback signals increasing to a predeterminedlevel when the vehicle is not following a guide line or the guidancesignal is not stronger than the signal produced by background radiationwhen the radiation means is switched on; and line detection means forpreventing the operation of the vehicle guidance means whenever at leastone of said error and feedback signals increases to said predeterminedlevel.